1. Technical Field
This invention relates to the field of seals used in rotating machinery to prevent the leakage of fluids. This invention relates more specifically to the abrasive components used in abrasive/abradable seals which prevent interaction between moving components in the aforementioned rotating machinery.
2. Background Information
Turbine and compressor sections within an axial flow turbine engine generally include one or more rotor assemblies each having a plurality of rotor blades circumferentially disposed around a disk rotating within a cylindrical case. For efficiency sake, each rotor assembly includes seals for sealing between the rotating members and the stationary members. The seals increase the efficiency of the engine by preventing the leakage of air where little or no work can be either imparted or extracted. Abradable seals, which include a "hard" abrasive component designed to contact a "soft" abradable component, are a popular choice for such seals. The abradable component generally consists of a brittle, frangible material that in theory breaks cleanly away when contacted by an abrasive component. The abrasive component, on the other hand, consists of a hardened, tough material that in theory will not yield during contact with the abradable component. In the case of the blade outer air seal, the abrasive component is typically applied to the blade tips and the abradable component is applied to the inner diameter of the case. Disparate thermal and/or dynamic growth between the rotor assembly and the case causes the abrasive component to contact the abradable component and thereby seal between the two components. The softer abradable component yields to the abrasive component and thereby prevents mechanical damage to either the blade tips or the case.
A disadvantage of abradable seals is that some compatible abrasive and abradable components perform best at high incursion rates, while others perform best at low incursion rates. The incursion rate between a rotating member and a structure radially outside of the rotating member reflects the frequency at which the rotating member strikes the structure and the magnitude of interference between the two at each pass. Very few abrasive and abradable components provide optimum performance at both high and low incursion rates. For example, it is known that ceramic particulate matter dispersed within a metal matrix may be used as an abrasive component. At low incursion rates, the particulate matter favorably operates as a plurality of minute cutters to "machine" a path within the abradable component. At high incursion rates, however, elevated temperatures can compromise the metal matrix and cause it to release the ceramic particulate matter. The degradation of the abrasive component creates a greater than optimum gap between the rotor and the case and thereby decreases the efficiency of the engine.
What is needed is a abrasive component for an abradable seal for a gas turbine engine that performs favorably at high and low incursion rates.